This invention relates to photography, and particularly to a novel film speed control system for sound motion pictures.
A classic problem in the art of recording and reproducing sound movies, of the kind in which a sound track is recorded along the movie film, is that the sound must be recorded and reproduced at the same uniform speed to avoid frequency distortion, commonly termed wow and flutter, whereas the film must be moved intermittently past the exposure or projection gate so that the sequence of still images can be properly exposed or projected. Various solutions to this problem have been proposed.
One proposed solution is the provision of a moving optical system which allows the film to move at a uniform speed through the film gate. So far as is known, this approach has not been widely adopted. It is characterized by the difficulty in designing an optical system including moving mirrors or the like, which will reproducibly capture properly spaced frames during exposure, and, more particularly, to recapture the frames in proper registration on projection. The matter is complicated considerably by the need to maintain correct focus and to avoid distortion in the projected image.
A more common approach is to provide for a loop of film between the sound station and the film gate in which each image is projected or exposed. As the film is started and stopped at the projection gate, the loop is shortened and lengthened to allow the film to be moved uniformly past the sound station by a capstan. In order to maintain the loop, it has been proposed to sense its length and to control the speed of either the capstan or of the incremental drive at the film gate so that the film loop has a constant average length.
Particularly in projectors, where the projection lamp represents a substantial power demand, it is highly desirable to avoid film speed control systems which result in any substantial regulation of the power supply voltage. In addition to the lamp, a power supply is required to drive the various constant speed or variable speed motors used in the projector, to energize the various electronic circuits that may be provided, and to drive the blower that is usually employed to cool the projection lamp and its surroundings. Many otherwise suitable motor speed control systems would either place onerous requirements on the power supply or would not produce a sufficiently constant speed past the sound transducer.
Projectors are commonly required to operate from a conventional alternating voltage line, which is frequently a 60 Hz, nominally 120 volt line. Particularly for small film formats such as the conventional 8 mm of Super 8 formats, a light source of sufficient brightness and appropriately small size usually has the longest service life when designed to operate at a voltage well below 120 volts. One such commonly available projection lamp, for example, requires, a voltage in the neighborhood of 20.5 volts AC for optimum performance.
It is, of course, quite possible to provide a separate transformer for operating the projection lamp. However, the large physical size, weight and cost of a transformer that will operate a 100 or 150 watt lamp at 20 volts from 120 volt source militates against its use.
One solution that has been proposed to this power supply problem involves the use of a combined motor and transformer in which the motor is of the shaded pole, induction type, and in which auxiliary windings on the ferromagnetic stator or the motor supply power for the projection lamp and other electrical components of the system. The rotor of this combined motor and transformer drives the blower for the system, and also drives the film advance claw of the projector through a reciprocating mechanism of conventional design. A DC motor drives the sound capstan, through a flywheel, if desired.
Because the utmost uniformity in the speed of the film past the capstan is essential to good sound quality, it would be highly desirable to drive the capstan motor at a constant speed, and to adjust the speed of the combination motor and transformer so that the average speed of the film moved by the claw would be the same as the speed of the film past the sound transducer. However, so far as I am aware, prior to my invention, speed control systems for induction motors have involved regulation of both the rms and the peak-to-peak values of the voltage applied to the motor windings. That is undesirable in projectors of the kind described, for two reasons. The first is that material variations of the rms voltage applied to the projection lamp will change its brightness enough to visibly affect the quality of the projected image. Secondly, any variation in the peak-to-peak voltage will tend to vary the DC voltage derived for controlling the constant speed motor unless an elaborate regulating scheme is provided.
One object of my invention is to facilitate the control of the shaft speed of a combined motor and transformer without affecting the peak-to-peak value, and without materially affecting the rms value, of the transformer voltage. Another object of my invention is to simplify the design and construction of sound motion picture projectors. A more general object is to provide an improved speed control circuit for induction motors.
Briefly, the above and other objects of my invention are attained by novel modifications in the construction of a projector of the type having constant speed drive means for advancing a strip of movie film bearing a magnetic sound track at a constant speed past a transducer, and having a projection station past which the film can be incrementally advanced for projection with light from a projection lamp. A drive claw for incrementally moving the film past the projection station is driven by the rotor of a combination shaded pole induction motor and transformer. A conventional DC power supply is energized from a secondary winding on the stator of the combination motor and transformer. Energy for the lamp is supplied by one of the windings on the stator.
The primary winding of the combination motor and transformer is energized over a novel control circuit which admits one half-cycle of the line voltage without appreciable attenuation, and admits most of the other half-cycle of the line voltage, that portion admitted being also without substantial attenuation. A portion of the controlled half-cycle of the line voltage, near the zero crossing of the line voltage, is not admitted to the primary winding. This portion is gated out by a control circuit under the control of a voltage responsive gate which has an effective time constant under the control of photocell.
The photocell senses the length of a loop in the film between the sound transducer and the film gate to selectively interrupt a portion of one half-cycle of the line voltage. This action induces a DC unbalance in the motor drive voltage which acts as a brake on the motor. Variable application of the braking force enables the motor to be controlled so that its average speed is fixed relative to the capstan speed. I have found that adequate regulation of the motor speed can be provided in this way without affecting the peak-to-peak value of the motor voltage, and with only very minor variations in the rms value of the transformer voltages.
The manner in which the apparatus of my invention is constructed, and its mode of operation, will best be understood in the light of the following detailed description, together with the accompanying drawings, of various illustrative embodiments.